1. DNA condenses to form chromosomes
4. each pair of chromosomes line up in the middle of the cell
3. pairs of chromosomes separate
2. A nuclear membrane forms around chromosomes
Explanation:
Prophase I
During prophase I, the chromosomes condense and become visible inside the nucleus. Because each chromosome was duplicated during the S phase that occurred just before prophase I, each now consists of two sister chromatids joined at the centromere. This arrangement means that each chromosome has the shape of an X.
Once this chromosomal condensation has occurred, the members of each chromosome pair (called homologous chromosomes, because they are similar in size and contain similar genes), align next to each other. At this point, the two chromosomes in each pair become tightly associated with each other along their lengths in a process called synapsis. Then, while the homologous chromosomes are tightly paired, the members of each pair trade adjacent bits of DNA in a process called crossing over, also known as recombination (Figure 1). This trading of genetic material creates unique chromosomes that contain new combinations of alleles.
At the end of prophase I, the nuclear membrane finally begins to break down. Outside the nucleus, the spindle grows out from centrosomes on each side of the cell. As in mitosis, the microtubules of the spindle are responsible for moving and arranging the chromosomes during division.
Metaphase I
A schematic shows two pairs of homologous chromosomes arranged in a vertical formation in the middle of a cell. Two developing mitotic spindles flank the four chromosomes. Long fibers radiating from the mitotic spindles are attached to the centromere of each chromosome.
At the start of metaphase I, microtubules emerge from the spindle and attach to the kinetochore near the centromere of each chromosome. In particular, microtubules from one side of the spindle attach to one of the chromosomes in each homologous pair, while microtubules from the other side of the spindle attach to the other member of each pair. With the aid of these microtubules, the chromosome pairs then line up along the equator of the cell, termed the metaphase plate
Anaphase I
A schematic shows each member in a pair of homologous chromosomes separating from its partner and migrating in opposite directions. White spindle fibers attached to the centromeres of each chromosome are responsible for the movement of the chromosomes to opposite poles of the cell.
During anaphase I, the microtubules disassemble and contract; this, in turn, separates the homologous chromosomes such that the two chromosomes in each pair are pulled toward opposite ends of the cell (Figure 3). This separation means that each of the daughter cells that results from meiosis I will have half the number of chromosomes of the original parent cell after interphase. Also, the sister chromatids in each chromosome still remain connected. As a result, each chromosome maintains its X-shaped structure.
Telophase I
A schematic shows two overlapping, circular cells, each with a nucleus and two chromosomes. The cytoplasmic portions of the cells are light grey, and the nuclei are dark grey. The chromosomes in the left-hand cell are mostly green, but the lower regions of the right chromatids are orange. The chromosomes in the right-hand cell are mostly orange, but the lower regions of the left chromatids are green. Both cells have remnants of the mitotic spindle in the cytoplasm.
As the new chromosomes reach the spindle during telophase I, the cytoplasm organizes itself and divides in two. There are now two cells, and each cell contains half the number of chromosomes as the parent cell. In addition, the two daughter cells are not genetically identical to each other because of the recombination that occurred during prophase I
